Intake structures for variable flow compressors frequently employ adjustable inlet guide vanes for imparting a varying swirl rate to the inlet air or gas. The swirl imparted by the inlet guide vanes has the effect of reducing the relative velocity between the inlet gas and the rotating compressor blades, thereby providing an effective method for modulating gas flow without changing the rotational speed of the compressor.
One particular type of intake structure for a radial compressor having an annular, axially facing inlet admits the inlet gas or air via a radially outward facing, circumferential intake opening The inlet gas flows radially inward through the intake and is turned axially by an annular gas flow passage defined between a radially inner hub wall and a radially outer wall. In this arrangement, the flow regulating inlet guide vanes are disposed circumferentially about the radially outward facing inlet and each vane pivots about a pivot axis generally parallel to the compressor shaft axis.
For maximum flow, the inlet guide vanes are arranged so as to each be aligned radially with respect to the compressor shaft axis, thereby admitting the inlet gas with a zero swirl angle. For reduced or modulated flow, the inlet guide vanes are turned in unison so as to impart a swirling motion to the inlet gas in the same angular direction as the rotating compressor, thereby reducing the relative velocity at the compressor inlet face and hence the gas flow.
At inlet guide vane angles of 45.degree. or greater with respect to the radius, a flow instability has been found to arise which is manifested as an extremely loud, audible tone having a frequency of approximately 500 Hz, depending on the intake size and flow rate. A sound pressure level in excess of 100 decibels has been measured outside the intake. Measurements show that the tone is generated by a rotating pressure wave inside the intake and is accompanied by a radial, redistribution of gas total pressure and total temperature from the normal uniform distribution to a non-uniform distribution. Such phenomena are identical to the flow phenomena associated with the Ranque-Hilsch effect as described, for example, by Kuroska et al "`Vortex Whistle`: An Unsteady Phenomenon in Swirling Flow and its Effects on Steady Flow Field", AIAA-81-0212, (1981).
It is extremely undesirable to operate a compressor intake, or any other turbomachine, in such a flow regime because the extremely loud tone is unacceptable for applications in or near personnel occupied locations.
Prior art compressor arrangements have utilized various means for suppressing such noise, as described in U.S. Pat. Nos. 4,436,481, 4,439,104, and 4,531,356 which provide one or more elongated tabs secured to the inlet guide vane cascade and extending radially inward into the gas flow passage immediately adjacent the gas inlet. The tabs in each of the above-identified patents are claimed to create a small zone of random turbulence in the radially outer portion of the vortex flow induced by the inlet guide vanes. As noted in each of the above references, the flow disruption is confined to the radially outer portion of the gas flow vortex and to one or more relatively small portions thereof. Such disruption apparently allows the intake device of the cited references to avoid the vortex whistle or Ranque-Hilsch effect.